1. Field of the Invention
The present invention relates in general to electrochemical devices, and, more particularly to a high power electrochemical device in bipolar configuration capable of providing instantaneously available high power. The high power device is based on a nonaqueous electrolyte system containing lithium salt. Anode and cathode are constructed from high rate, highly stable and low cost materials and are selected to provide maximum device safety. Additionally, the present invention relates to the number of voltage-monitored cells connected in series in a bipolar configuration, in combination with the anode-to-cathode capacity ratio and with other device parameters, in order to provide an overall low-cost system having excellent cycle life and outstanding power performance.
2. Prior Art
The need for power quality is ever increasing. Current and future sophisticated electric and electronic devices are, and will continue to be, increasingly sensitive to power supply issues. For example, poor power supply and transient power losses in supplied main power can have an adverse impact on electronic equipment. Such power issues can result in the destruction of electronic equipment, the loss of generated data, the loss of communication with other equipment, and the loss of time required to reset and restart procedures that were interrupted by the power failure. Certain solutions have been developed to provide instantaneous power in response to transient power interruptions and other associated power problems, including uninterruptible power supply (UPS) systems. In such systems, lead-acid batteries are often used to provide temporary power when necessary. While such solutions have had some success, lead-acid batteries have certain problems, including, but not limited to, unsatisfactory cycle life, a high failure rate, high cost of maintenance, high weight, large size, toxicity of the battery materials, risk of hydrogen leaks posing a safety hazard, self discharge issues, and sensitivity to deep discharge and to temperature.
Another important area of application for high power electrochemical devices is in the automotive industry for hybrid electric vehicles and for a wide range of power assist functions. In order to reduce air pollution, particularly in congested cities, fuel efficiency needs to be further increased. This objective can be achieved by battery systems capable of providing instantaneous power for acceleration and hill climbing and the capability for regenerative braking. Such battery systems have to be able to provide and/or to absorb significant amounts of power over the relatively short period of a few seconds only. In addition, there is an increasing array of electrically powered ancillary systems in modern cars, e.g. for preheating catalytic converters, for electric brake or steering force amplifiers, drive. by wire, electrically controlled shock absorbers, and the like.
These and other applications for industries including, but not limited to, car manufacturers, providers of public transport, power quality, power back-up systems, and remote area power supplies in combination with wind turbines, photovoltaic cells and/or an electricity generator based on fossil fuels such as diesel or natural gas, will increasingly require compact and low-cost storage systems capable of delivering large amounts of electrical power from a limited volume. In order to fulfill requirements for such power systems, the present invention is directed to an electrochemical energy storage device, such as a rechargeable battery device, wherein both the cathode and the anode are constructed from high rate, highly stable and low cost materials and assembled in a bipolar configuration. While battery devices exist in the older battery technologies of lead-acid, nickel-cadmium, and nickel-metal hydride, large high power Li-ion batteries have not been introduced in significant quantities to the market place yet, mainly due to safety considerations and due to the requirement of single cell control. Electronic control of each cell would add considerably to the cost and the complexity of high voltage, high power back-up or power assist systems based on standard Li-ion batteries. It is therefore an object of this invention to provide a means whereby electronic control can be drastically reduced.
A Li-ion battery comprising lithiated titanate anodes, manganese oxide-based cathodes, and a poly(acrylonitrile) polymer electrolyte has been described, for example in U.S. Pat. No. 5,766,796. Such a configuration is however not in the spirit of the present invention since the capacity utilization at rates being significantly larger than 10C is too low for high power applications. Battery discharge and charge currents are often given in “C”-rate. 10C corresponds to ten times the amount of current in relation to the amount of battery capacity (e.g. a 10 Ampere current for a 1 Ampere hour battery). A bipolar Li-ion battery device with excellent high power performance, stability, and safety characteristics has recently been disclosed in PCT Application “rechargeable High Power Electrochemical Device.” Those skilled in the art will appreciate that a bipolar configuration with a maximum number of cells in series leads to a battery with maximum energy density and a lower cost for electronic monitoring of the battery. On the other hand, too many cells in series without electronic monitoring may limit battery life because slight differences from cell to cell may lead to voltage differences during charge and discharge. If one or several cells are continuously slightly overcharged battery life may be reduced. No optimum number of cells connected in series without electronic monitoring can be given from first principles. We discovered that there is a complex and unexpected interrelationship between battery life and battery design parameters including, but not limited to, the number, m, of cells in a voltage-monitored subgroup in a bipolar battery, anode-to-cathode capacity ratio, and electrolyte properties such as conductivity. It is, therefore, an object of this invention to provide key features enabling a high power battery in bipolar configuration to have long cycle life and excellent power performance over tens of thousands of discharge and charge pulses while minimizing the cost for electronic monitoring.
These and other objectives will become apparent in light of the specification and claims appended thereto.